Generally, a bipolar junction transistor improves in current performance, speed, and gain as compared to a metal oxide semiconductor transistor, and is thus widely used when designing an analog power RF IC.
A bipolar junction transistor having an emitter, a base, and a collector is classified into a vertical bipolar junction transistor and a lateral bipolar junction transistor depending on the traveling direction of electric charges emitted from the emitter.
The bipolar junction transistor is used as an electrostatic discharge protection element which protects an internal IC circuit.
FIG. 1 is a sectional view illustrating a lateral bipolar junction transistor as an electrostatic discharge protection element in the related art. FIG. 1 shows an npn-type bipolar junction transistor.
As illustrated in FIG. 1, a bipolar junction transistor for an electrostatic discharge protection element in the related art includes a first trench element isolation film 14, a second trench element isolation film 15, and a third trench element isolation film 16 which are formed on a p-type semiconductor substrate 11, and define an emitter region, a base region, and a collector region.
A p-type impurity is implanted into the emitter region and the base region including the first trench element isolation film 14 of the semiconductor substrate 11 to form a first well 12.
An n-type impurity is implanted into the collector region including the third trench element isolation film 16 of the semiconductor substrate 11 to form a second well 13.
A p-type impurity is implanted into the base region between the first trench element isolation film 14 and the second trench element isolation film 15 to form a base 19.
An n-type impurity is implanted into the collector region between the first well 12 and the third trench element isolation film 16 to form a collector 18.
An n-type impurity is implanted into the emitter region isolated from the base region by the first trench element isolation film 14 to form an emitter 17.
When the bipolar junction transistor is used as an electrostatic discharge protection element, an input/output (I/O) terminal should be constituted at the time of circuit design. The collector is connected to the input/output terminal, the emitter is connected to the ground terminal, and the base is connected to the ground terminal through a resistor.
The bipolar junction transistor for an electrostatic discharge protection element is used to protect a semiconductor device from static electricity. If an electrostatic voltage equal to or higher than 2000 V is input to the input/output terminal, the bipolar junction transistor rapidly discharges an electrostatic current to the ground terminal. For this reason, in order to use the bipolar junction transistor as an electrostatic discharge protection element, the parameters, such as a triggering voltage Vt, a holding voltage Vh, and a breakdown voltage Vb, must be satisfied.
On the other hand, a lateral bipolar junction transistor exhibits unsatisfactory characteristics as compared to a vertical bipolar junction transistor from the viewpoint of electrostatic discharge protection performance.
Accordingly, in the related art, as illustrated in a sectional view of FIG. 2, the positions of the emitter 17 and the base 19 are reversed. As illustrated in FIG. 2, if the positions of the emitter and the base are reversed, the tunneling effect of the bipolar junction transistor appears faster than that of FIG. 1, such that the triggering time can be advanced.
However, even when the positions of the emitter and the base are reversed, it is difficult to secure a satisfactory electrostatic discharge protection. For this reason, the size of the bipolar junction transistor must still be increased.
The increase in the size of the bipolar junction transistor leads to an increase in the size of the semiconductor device. Accordingly, there is a limit to which the size of the bipolar junction transistor may be increased before it becomes undesirable.